Doing it in one loop:

#!/usr/bin/perl -w
use strict;

die "Usage: ", $0 =~ /([^\/]+)$/, " <filename>\n" unless @ARGV;

$^I = '';                       # We're really brave - no backup file!
+ 

while (<>){
    next unless /./;            # Skip empty lines
    tr/A-Z/a-z/;                # Lowercase the lot
    s#^(?!http://)#http://#;    # Prefix 'http://' if one is missing
    print;
}
[download]

If 'petpages' contains the following:

foobar.com
http://foo.int
fOoBaR.nEt
FoObAr.OrG
[download]

then running the above script with 'petpages' as an argument results in the content being changed to this:

http://foobar.com
http://foo.int
http://foobar.net
http://foobar.org
[download]

Update: Clarified phrasing a bit.